Disposal of Municipal Solid Waste (MSW) and Municipal Solid Sludge (MSS) are significant issues throughout the world, and especially in the developed world. The traditional techniques of either burying or incinerating MSW and MSS are resulting in significant problems. Landfills are increasingly running out of space and there is becoming a large requirement to truck huge amounts of MSW/MSS to distant locations due to the public's unwillingness to have landfills in their neighborhood.
The environmental impact of dumping the MSW and MSS and/or incinerating it in a traditional fashion are enormous with toxins leaching into the soil surrounding landfills and potentially carcinogenic elements entering the air during incineration. The public interest in environmentally acceptable solutions is growing and the push has been in most developed countries to Reduce, Reuse and Recycle in order to limit the MSW that makes it to the landfills and reduce the energy used in dealing with it.
In some situations, benefits have been gained during the processing of MSW and MSS. During incineration, there is often reuse of the heat generated in order to create electricity or heat one or more facilities. In landfills, there have been successful attempts to capture methane that is released in the breakdown of the MSW over time.
This methane can then be used in a combustion chamber to create heat energy or within a chemical process to form more complicated compounds. The problem is these solutions do not solve the underlying environmental problems and do not come close to properly capturing the energy within the MSW and MSS.
One technology that has been developed to better process MSW is called plasma arc gasification. In plasma arc gasification, a plasma arc is generated with electrical energy in order to reduce complex carbon-containing molecules into smaller constituent molecules. This molecular breakdown occurs without the presence of oxygen, ensuring that combustion does not occur. The process uses pyrolysis to molecularly breakdown the complex carbon compounds into simpler gas compounds, such as carbon monoxide CO and carbon dioxide CO2, and solid waste (slag). The process has been intended to reduce the volumes of MSW being sent to landfill sites and to generate syngas, a useful gas mixture, as an output.
Syngas describes a gas mixture that contains varying amounts of hydrogen H2, carbon monoxide CO, and carbon dioxide CO2, generated through the gasification of a carbon-containing compound. Syngas is combustible, though with typically less than half the energy density of natural gas. It is used as a fuel source or as an intermediate product for the creation of other chemicals. When used as fuel, coal is often used as the source of carbon by the following reactions:C+O2→CO2 CO2+C→2COC+H2O→CO+H2 This is a mature technology that has seen a renewed interest as a cleaner method of combusting coal than the traditional use of solid coal. When used as an intermediate product in the production of other chemicals such as ammonia, natural gas is typically used as the feed material, since methane has four hydrogen atoms which are desirable for syngas production and methane makes up more than 90% of natural gas. The following steam reforming reaction is used commercially:CH4+H2→CO+3H2 The traditional syngas generation technologies using coal and natural gas as feed inputs differ from plasma arc gasification in that they occur within a controlled oxygen environment whereas the plasma arc gasification occurs in an oxygen-free environment. Though designated oxygen-free, through the molecular breakdown of input material, there will be the production of small quantities of oxygen within the process. Further, the coal and natural gas techniques use consistent input materials which results in consistent syngas composition, while plasma arc gasification implementations to date typically use MSW as input material in which feedstock variability leads to syngas variability.
Unfortunately, thus far, there have been no municipal scale implementations of plasma arc gasification due to a number of limiting aspects of the technology. Firstly, most implementations of the technology have not been designed to manage the high flow rate of MSW that would be required in a commercial facility. Further, the pyrolysis techniques used have led to high levels of contaminant compounds such as tars, rather than the full conversion to hydrogen H2, carbon monoxide CO, carbon dioxide CO2 and hydrocarbons (C1 to C4s). The inconsistent nature of the MSW input material has led to high variability in the quality of the generated syngas. Yet further, high levels of energy are consumed in the creation of the plasma arc and, in some instances, in drying the MSW prior to processing due to moisture limits on the input materials, while the generated syngas has a low calorific value, typically less than half of the BTU content of natural gas. These concerns have limited this technology, despite the significant benefits of converting MSW into a valuable product such as syngas.
Against this background, there is a need for solutions that will mitigate at least one of the above problems, particularly enabling the generation of syngas from input material such as MSW and/or MSS in an efficient manner.